The present invention relates to a bottom rail for a covering for an architectural opening such as Venetian blinds, pleated shades, and other blinds and shades. Typically, a blind transport system will have a top head rail which both supports the blind and hides the mechanisms used to raise and lower or open and close the blind. The raising and lowering is done by lift cords which support the bottom rail (or bottom slat). This bottom rail is normally heavier and larger in cross-section, or more rigid, than any of the slats that are intermediate between it and the head rail. The blind may be tilted in the forward direction and in the rear direction. The tilting is typically accomplished with ladder tapes (and/or tilt cables) which run along the front and back of the blind and are also attached to the bottom rail. By shortening one of the tilt cables relative to the other, the corresponding edge of the blind is lifted up, causing the blind to tilt upwardly in the direction of the shortened tilt cable and downwardly in the direction of the extended tilt cable. The lift cords (in contrast to the tilt cables) may run along the front and back of the stack of slats or through slits in the middle of the slats, and are connected to the bottom rail.
In these constructions, the closure of the blinds (tilting closed) tends to become less effective toward the bottom of the blind. When the blind is fully lowered, all the weight has been lifted off of the lift cords and transferred to the ladder tapes containing the tilt cables. This enables the ladder tapes to have the maximum influence on tilting the bottom rail, which tends to maximize the closure at the bottom of the opening. However, even then, while the shortened cable adjacent to the edge of the blind which is tilted upwardly is under tension, the edge of the blind which is tilting downwardly is under no tension except what little tension gravity can afford, since the tilt cables can only function under tension, but not under compression (you cannot push on a rope). This gravitational influence on the downwardly tilting edge of the blind is partially offset by the ladder tapes, which take some of the weight of each slat away from the extended tilt cable and transfer it to the shortened tilt cable. Thus, the shortened tilt cables support more of the weight and, as a result, tend to stretch more, while the extended cables support less of the weight and thus tend to stretch less. This often results in incomplete closure of the blind.
This situation is aggravated for a product in which the lift cords run along the front and back of the stack of slats. In this instance, when the blind is fully lowered, once again all the weight has been lifted off of the lift cords and transferred to the ladder tapes. However, as soon as the tilting action is started, the edge of the blind which is tilted upwardly is free to rise, but the opposite edge is not free to go downwardly, because, as soon as it starts to do so, it encounters interference from the lift cable. This stops the downward movement of that tilting edge, and the bottom rail stops pivoting around its center and instead begins to pivot about its now fixed, downwardly tilting edge, therefore lifting the center of gravity of this bottom rail and causing poor closure. Thus, in this type of product, the poor closure is due both to a lack of tension on the ladder tapes on the downwardly tilting edge of the bottom rail, and to the interference by the lift cords with the downward motion of the downwardly tilting edge.
The Swedish Patent application SE 15427/64 (filed on Dec. 19, 1964) attempts to address this incomplete closure problem by installing a free rolling weight in the bottom rail. As the bottom rail is tilted, the free rolling weight shifts to one edge of the bottom rail, thus putting the extended tilt cable under increased tension caused by the shifting weight. However, this solution does nothing to alleviate the problem caused by the interference by the lift cords with the downward motion of the downwardly tilting edge in the situation where the lift cords run along the front and back of the stack of slats.
One example of an embodiment of the present invention provides a bottom rail with a shifting weight and lift cords which support the bottom rail while being free to move in the forward-to-rear direction relative to the bottom rail. In this arrangement, the shifting weight in the bottom rail moves to whatever edge is the downwardly tilting edge of the bottom rail and thus, by increasing the weight at that edge, aids in putting the extended tilt cables under tension, enhancing the closure of the blind. Furthermore, because the bottom rail is free to move in the front-to-back direction relative to the lift cords, the lift cords do not interfere with the tilting of the blind. Thus, the blind closes properly, even at the bottom.